Method for Manufacturing High Strength Ultra-Fine/Nano-Structured Al/Aln or Al Alloy/Aln Composite Materials

ABSTRACT

The present invention relates to the a method for manufacturing high strength ultra-fine/nano-structured aluminum/aluminum nitride or aluminum alloy/aluminum nitride composites using mechanical milling or mechanical alloying process which is conducted in the nitride-forming atmosphere such as nitrogen gas (N), ammonia gas (NH) or mixed gas including both gases, subsequent heat treatment process, and hot consolidation process. Also, high strength ultra-fine/nano-structured Al/ALN or Al alloy/ALN composite materials fabricated by the method of present invention have superior mechanical strength and heat resistance to those fabricated by conventional powder metallurgy process or liquid processes.

TECHNICAL FIELD

The present invention relates to high strengthultra-fine/nano-structured aluminum/aluminum nitride or aluminumalloy/aluminum nitride composite materials, and their manufacturingprocess.

BACKGROUND ART

Currently aluminum matrix composite materials reinforced with aluminumoxide, silicon carbide, and etc show poor interface characteristics,such as decohesion of re-inforcements, reaction with matrix phases, andpoor wettability. These problems limit the usage of aluminum matrixcomposite materials. Aluminum nitride is a potential re-inforcement foraluminum matrix composite materials having high elastic modulus, highthermal conductivity, good thermal stability, chemical stability andsound wettability with aluminum.

However, conventional processes, such as conventional powder metallurgyprocess, directed melt nitridation process, and liquid process in whichaluminum nitride reinforcements are directly added to aluminum melts,show some problems like poor interface characteristics caused by theoxide layer of aluminum nitride particles, inhomogeneous distribution ofreinforcements, and difficulty in controlling the reinforcement size.Therefore, the a novel method for the fabrication of aluminum/aluminumnitride or aluminum alloy/aluminum nitride composite materials havinghomogeneous distribution of fine reinforcement particles and preventionof oxide layer at the surface of matrix and reinforcements should bedeveloped.

DISCLOSURE OF INVENTION Technical Problem

An object of the invention is to provide high strengthultra-fine/nano-structured aluminum/aluminum nitride or aluminumalloy/aluminum nitride composite materials exhibiting superiorinterfacial characteristics, and their manufacturing method, which iscomposed of mechanical milling or mechanical alloying process which isconducted in the nitride-forming atmosphere such as nitrogen gas (N₂),ammonia gas (NH₃) or mixed gas including both gases, subsequent heattreatment process, and hot consolidation process. Following According tothe present invention, the direct nitride forming reaction is occurredin conventional mechanical milling or mechanical alloying process whichis currently used. Therefore, the present invention offers the aneconomical method for manufacturing a high strengthultra-fine/nano-structured aluminum/aluminum nitride or aluminumalloy/aluminum nitride composite materials, because no additionalprocess is needed.

The continuous supply of nitride-forming gases, such as nitrogen gas(N₂), ammonia gas (NH₃) or mixed gas including both gases, and theremoval of residual products after nitride-forming reaction is arecrucial for the present invention. The gas supplying devices, gasregulator, and flow rate controller are attached to a milling device sothat the pressure of milling chamber and the flow rate ofnitride-forming gases are maintained constantly.

ADVANTAGEOUS EFFECTS

As apparent from the above description, the present invention providesaluminum/aluminum nitride or aluminum alloy/aluminum nitride compositematerials having superior mechanical strength at ambient and elevatedtemperature, and heat resistance to those fabricated by conventionalpowder metallurgy process or liquid processes, and their fabricationmethod using mechanical milling/alloying process.

Although the preferred embodiments of the invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the inventions as disclosed inthe accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the schematic diagram of the present invention.

FIG. 2 is a graph depicting the result of X-ray diffraction pattern ofaluminum powders, which are fabricated by mechanical milling/alloyingprocess in nitride-forming atmosphere, and of powders which are heattreated at 500° C., 600° C., and 900° C. for 1 hour.

FIG. 3 is a graph depicting the result of differential thermal analysisof aluminum powders which are milled in nitride-forming atmosphere.

FIG. 4 is a graph depicting the results of X-ray photoelectronspectroscopy of aluminum powders, which are fabricated by mechanicalmilling/alloying process in nitride-forming atmosphere, and of powderswhich are heat treated at 500° C., and 600° C., and 900° C. for 1 hour.

FIG. 5( a), 5(b), 5(c), and 5(d) are transmission electron micrographsof aluminum/aluminum nitride composite powders, which are fabricated bymechanical milling/alloying and subsequent heat treatment: (a) brightfield image of composite powders, (b) dark field image of aluminum incomposite powders, (c) dark field image of aluminum nitride in compositepowders, and (d) selected area electron diffraction pattern of compositepowders.

FIG. 6 is a graph depicting the variation of hydrogen contents inaluminum/aluminum nitride composite powders, which are fabricated bymechanical milling/alloying process, during subsequent heat treatment at540° C. and 560 580° C.

BEST MODE FOR CARRYING OUT THE INVENTION

Mechanical milling or mechanical alloying process used in the presentinvention is a solid-state powder processing technique involvingrepeated welding, fracturing, and the rewelding of powders due torepeated collisions between milling media, such as steel balls andceramic balls, and powders in milling devices like a ball mill and anattritor. Mechanical milling or mechanical alloying process has a numberof advantages including grain refinement, homogeneous distribution ofreinforcement, extended solid solubility, and the formation ofmetastable or amorphous phases, depending on the initial powdercomposition and processing condition. For the effectiveness ofmechanical milling or mechanical alloying process, it is appropriatethat the initial charge ratio of milling media to powders is 5:1 to50:1.

After mechanical milling or mechanical alloying process innitride-forming atmosphere, the precursor of aluminum nitride is formed.To transform this precursor into aluminum nitride, subsequent heattreatment process is needed.

The conditions of subsequent heat treatment process of composite powdersshould be acquired from the results of various experiments which aredifferential thermal analysis. X-ray photoelectron spectroscopy, X-raydiffraction analysis, isothermal heat treatment and hydrogen analysis.Subsequent heat treatment process to transform the precursors ofaluminum nitride into aluminum nitride phase should be conducted attemperatures above of 400° C. or above, which is lower than the meltingpoint of composite powders, during 0.1 to 48 hours, or is substitutedfor degassing process. Naturally, degassing process is conducted atconditions similar to those of heat treatment process.

The aluminum/aluminum nitride and aluminum alloy/aluminum nitridecomposite powders fabricated by the process previously described isconsolidated by the process such as hot pressing, hot isostaticpressing, hot extrusion, and etc. The hot consolidation processes ofpowder materials require the degassing process which removes residualmoisture, organic compounds and hydrogen. It is the necessary processfor improving mechanical properties of final consolidated materials. Itis composed of a canning process of powder materials, a sealing processof metal can and a evacuating process at high temperatures (below 1 10⁻¹torr).

In the present invention, subsequent heat treatment process issubstituted for the degassing process, which is conducted with under theconditions of subsequent treatment process, at temperatures above of400° C. or above, which is lower than the melting point of compositepowders, during 0.1 to 48 hours. Therefore, no additional process isneeded.

The present invention gives the method for manufacturing theultra-fine/nano-structured aluminum/aluminum nitride or aluminumalloy/aluminum nitride composite materials, in which the grain size ofaluminum matrix or aluminum alloy matrix, and the size of aluminumnitride reinforcements is below 100 particles are 100 or below.

Also, for improving the mechanical properties of aluminum matrix, suchas strength and hardness, solid solution hardening elements likemagnesium, silver and manganese could be added in the range of with 0.1wt. % to below solubility limit; precipitation hardening elements likecopper, zinc, silicon, titanium, iron, lithium, tin, chromium andzirconium could added with above solubility limit or above, rare earthelements like yttrium, cerium, lanthanum, scandium, samarium, neodymium,gadolinium, praseodymium and misch metal could be added with in therange of 0.1 wt. % to 10.0 wt. %; alloying elements like tungsten,molybdenum and cobalt could be added with in the range of 0.1 wt. % to50.0 wt. %; or ceramic reinforcement particles like Al₂O₃, SiC and Si₃N₄could be added with in the range of 0.1 wt. % to 50.0 wt. % in thepresent invention.

The present invention is described with following figures in detail inreference to the figures.

FIG. 1 is the schematic diagram of the present invention. In the step ofpowder preparation (P1), the powder of aluminum and alloying elements isprepared. Alloying elements is added in the form of individual elementpowders or master alloy powders. In the step of mechanical milling ormechanical alloying in nitride-forming atmosphere (P2), the mechanicalmilling or mechanical alloying is conducted for the nitride formation.The atmosphere in the milling container is composed of nitride-forminggases, such as nitrogen gas (N₂), ammonia gas (NH₃) or mixed gasincluding both gases. The devices for the continuous supply ofnitride-forming gases and the removal of residual products afternitride-forming reactions are attached. The step of the subsequent heattreatment process for the formation of aluminum nitride (P3) issubstituted for the degassing process in the present invention. In thestep of hot consolidation (P4), the composite powders is areconsolidated into the form of bulk products.

FIG. 2 is a graph depicting the results of X-ray diffraction pattern ofaluminum powders, which are fabricated by the mechanicalmilling/alloying process in nitride-forming atmosphere, and of powderswhich are heat treated at 500° C., 600° C., and 900° C. for 1 hour. Noaluminum nitride peak is observed in the as-milled powders and thepowders which are heat treated at 500° C. The powders which are heattreated at 600° C. show evidence of aluminum nitride formation. Thealuminum nitride peaks are more sharp and obvious in the powders heattreated at 900° C. From this result, it is clear that the precursor ofaluminum nitride is formed in as-milled powders and a suitable heattreatment process is needed.

FIG. 3 is a graph depicting the result of differential thermal analysisof aluminum powders which are milled in nitride-forming atmosphere. Twoexothermic peaks are observed at 430° C. and 565° C. It is thought thatthese peaks are related to the trans-formation of aluminum nitrideprecursor into aluminum nitride. To verify the binding status ofnitrogen atoms, X-ray photoelectron spectroscopy is conducted.

FIG. 4 is a graph depicting the results of X-ray photoelectronspectroscopy of aluminum powders, which are fabricated by mechanicalmilling/alloying process in nitride-forming atmosphere, and of powderswhich are heat treated at 500° C., and 600° C., and 900° C. for 1 hour.It is found that nitrogen atoms have Al—N, N—H and N—H₂ type bonds inas-milled powders, N—H₂ type bonds are disappeared through isothermalheat treatment at 500° C., and only Al—N type bonds is existing afterisothermal heat treatment above 600° C.

FIG. 5( a), 5(b), 5(c), and 5(d) are transmission electron micrographsof aluminum/aluminum nitride composite powders, which are fabricated bymechanical milling/alloying and subsequent heat treatment. Those showthat the size of aluminum grains and aluminum nitride phases is below200 nm. The composite powders have ultra-fine structure ornanostructure.

FIG. 6 is a graph depicting the variation of hydrogen contents inaluminum/aluminum nitride composite powders, which are fabricated bymechanical milling/alloying process, during subsequent heat treatment at540° C. and 560 580° C. In the case of 540° C., no hydrogen is detectedafter 3 hours and in the case of 560 580° C., no hydrogen is detectedafter 2 hours. This These results show that the precursor of aluminumnitride in as-milled powders is transformed into aluminum nitridecompletely.

Mode for the Invention

Now, the high strength ultra-fine/nano-structured aluminum/aluminumnitride or aluminum alloy/aluminum nitride composite materials accordingto the present invention will be described in more detail, withreference to the following examples.

Example 1

In accordance with the present invention, the mixture of aluminumpowders and master alloy powders with the composition of Al-50 wt. % Mgare used as starting materials. Initial composition of startingmaterials is Al-4 wt. % Mg. Then mechanical alloying process isconducted in ammonia gas atmosphere with the condition in which thefinal volume fraction of aluminum nitride is 25%. The final bulk productof Al-4 wt. % Mg/25 vol. % AlN composite materials is manufacturedthrough cold compacting process, degassing process and hot extrusionprocess.

Example 2

In accordance with the present invention, the mixture of aluminumpowders and titanium powders are used as starting materials. Initialcomposition of starting materials is Al-5 wt. % Ti. Then mechanicalalloying process is conducted in ammonia gas atmosphere with thecondition in which the final volume fraction of aluminum nitride is 25%.The final bulk product of Al-5 wt. % Ti/25 vol. % AlN compositematerials is manufactured through cold compacting process, degassingprocess and hot extrusion process.

Example 3

In accordance with the present invention, the mixture of aluminumpowders and zinc powders are used as starting materials. Initialcomposition of stalling materials is Al-5 wt. % Zn. Then mechanicalalloying process is conducted in ammonia gas atmosphere with thecondition in which the final volume fraction of aluminum nitride is 25%.The final bulk product of Al-5 wt. % Zn/25 vol. % AlN compositematerials is manufactured through cold compacting process, degassingprocess and hot extrusion process.

The compression tests are conducted at room temperatures and 200° C.,and their results are shown in table 1. From examples 1 to 3, thealuminum alloy/aluminum nitride composite materials have superior yieldstrength to those of conventional aluminum alloys and aluminum matrixcomposite materials. Especially, Al-4 wt. % Mg/25 vol. % AlN compositematerial shows the drastic increase of yield strength at roomtemperature. These results of the aluminum/aluminum nitride compositesis are due to the refinement of aluminum matrix grains and reinforcementparticles and the homogeneous distribution of aluminum nitrideparticles.

TABLE 1 Comparison of Yield Strength of Various Samples Yield StrengthYield Strength at 25° C. at 200° C. Example Compositions (MPa) (MPa) 1Al-4 wt. % Mg/ 957 328 25 vol. % AlN 2 Al-5 wt. % Ti/ 390 241 25 vol. %AlN 3 Al-5 wt. % Zn/ 440 225 25 vol. % AlN Comparative 330 Example comparative example: Min Zhao, Gaohui Wu, Dezhi Zhu, Longtao Jiang andZuayong Dou: Materials Letters, 58(2004) p. 1899.

INDUSTRIAL APPLICABILITY

As apparent from the above description, the present invention providesaluminum/aluminum nitride or aluminum alloy/aluminum nitride compositematerials having superior mechanical strength at ambient and elevatedtemperature, and heat resistance to those fabricated by conventionalpowder metallurgy process or liquid processes, and their fabricationmethod using mechanical milling/alloying process.

Although the preferred embodiments of the invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the inventions as disclosed inthe accompanying claims.

1. Method for manufacturing high strength ultra-fine nano-structuredaluminum and aluminum nitride or aluminum alloy and aluminum nitridecomposite materials, wherein aluminum powder or mixture powder of thealuminum and alloying element is mechanically milled and mechanicallyalloyed in an atmosphere of NH₃ gas containing nitrogen for inducingnitrification reaction so as to produce a precursor of aluminum nitride;subsequent heat treatment is performed to produce aluminum and aluminumnitride or aluminum alloy and aluminum nitride composite powder; thiscomposite powder is subjected to a hot forming process to producecomposite material.
 2. The method of claim 1, wherein the mechanicalmilling and mechanical alloying are performed in a vessel in which theatmosphere of NH₃ gas containing is maintained.
 3. The method of claim1, wherein the subsequent heat treatment for production of aluminumnitride is performed at a temperature of 400° C. to the meltingtemperature of the composite powder for 0.1˜48 hours.
 4. The method ofclaim 1, wherein the subsequent heat treatment in the hot formingprocess is a high-temperature degassing process.
 5. The method of claim1, wherein the grains of aluminum or aluminum alloy matrix and thealuminum nitride reinforcement particles have a ultra-finenano-structure with a size equal to 10 μm or less through the mechanicalmilling or mechanical alloying process.
 6. The method of claim 1,wherein to aluminum matrix, solid solution hardening elements likemagnesium, silver and manganese could be added in the range of 0.1 wt. %to solubility limit, precipitation hardening elements like copper, zinc,silicon, titanium, iron, lithium, tin, chromium and zirconium couldadded with solubility limit or above, rare earth elements like yttrium,cerium, lanthanum, scandium, samarium, neodymium, gadolinium,praseodymium and misch metal could be added in the range of 0.1 wt. % to10.0 wt. %, alloying elements like tungsten, molybdenum and cobalt couldbe added in the range of 0.1 wt. % to 50.0 wt. % or ceramicreinforcement particles like Al₂O₃, SiC and Si₃N₄ could be added in therange of 0.1 wt. % to 50.0 wt. %.